Expose short commit sha's

Normally, many tools (docker, git etc) only show a short form of a long SHA. The $CI_COMMIT_SHA is a prime candidate for also having a short version available. While we can shorten it in the .gitlab-ci.yml file with something like echo $CI_COMMIT_SHA | cut -c 1-14 it does not give the granularity git itself for example gives us. (e.g. as short as possible). For git there's probably a command that produces the shortened output.

As a user, I'd also like to use these shorter SHA's, for example to tag a docker image with. Therefore I suggest adding a $CI_COMMIT_SHORT_SHA to generate the more convenient shorter but compatible tags.